Method of accessing the circuitry on a semiconductor substrate from the bottom of the semiconductor substrate

ABSTRACT

An apparatus is disclosed. In one embodiment, the apparatus includes a semiconductor substrate and a second substrate. The semiconductor substrate has a top side and a bottom side. The semiconductor substrate has an integrated circuit and at least one alignment fiducial formed on the top side. The alignment fudicial is aligned with the integrated circuit and the alignment fiducial is accessible from the bottom side. The semiconductor substrate further includes a first set of bond pads on the integrated circuit, the bond pads on the top side. The second substrate has a second set of bond pads corresponding to the first set of bond pads. The semiconductor substrate is coupled to the second substrate at a plurality of solder interconnections disposed between the first and the second set of bond pads.

This application is a continuation of application Ser. No. 08/724,223,filed on Oct. 2, 1996, now U.S. Pat. No. 5,952,247, which is acontinuation of application Ser. No. 08/344,149, filed on Nov. 23, 1994,abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuit testingand more particularly to a method of probing an integrated circuit.

BACKGROUND OF THE INVENTION

Once a newly designed integrated circuit (IC) has been formed on asilicon substrate, the IC must be thoroughly tested to be sure that thecircuit performs as intended. Any portion of the IC which does notfunction properly must be identified so that it can be fixed bymodifying the design of the IC. This process of testing an IC toidentify problems with its design is known as debugging. After debuggingthe IC and correcting any problems with its design, the final, fullyfunctional IC designs are used to mass produce the IC's in amanufacturing environment for consumer use.

During the debugging process, it is often necessary to probe certainelectrical interconnect lines in order to obtain important electricaldata from the IC, such as, for example, voltage levels, timinginformation, current levels, and thermal information. A typical ICdevice contains multiple layers of metal interconnects. However, themetal interconnects in the first metal layer of an IC device generallycarry the most valuable electrical data for debugging purposes. Metalinterconnect lines in the first metal layer reside closest to thesilicon substrate and are usually directly coupled to importantcomponents of the IC device such as transistors, resistors, andcapacitors. It is the electrical data received, manipulated, andtransmitted by these components which a designer is most interested inanalyzing during the debugging process.

FIG. 1a illustrates a surface view of the top side of an IC device.Metal interconnect lines and components of IC device 11 have been formedon an underlying silicon substrate. The side of the silicon substrateupon which the IC is formed shall herein be referred to as the top sideof the silicon substrate. As illustrated in FIG. 1a, bond pads 13 arelocated along the periphery of IC device 11. In the center of IC device11 is the active region 12 containing the majority of the high density,active circuitry of IC device 11. It is within active region 12 thatmost probing takes place during the debugging process. While probing theinterconnect lines in active region 12, it is necessary to externallysupply the proper voltage signals to bond pads 13 to activate thecircuitry within the active region. These voltage signals are suppliedto bond pads 13 through a package to which IC device 11 is affixed.

FIG. 1b illustrates a cross-section of IC device 11 after beingpackaged. After IC device 11 is affixed to package substrate 15,individual bond wires 14 are used to electrically couple each bond pad13 to a corresponding pad on package substrate 15. Each correspondingpad on package substrate 15 is then individually coupled to an externalpin 16. The packaged IC device of FIG. 1b may then be placed within asocket in order to electrically couple external pins 16 to drivers whichsupply the necessary voltage signals to activate IC device 11. Asillustrated in FIG. 1b, IC device 11 is mounted to package substrate 15with its top side facing away from package substrate 15. In this manner,once IC device 11 is activated through package pins 16, the internal,active region 12 may be accessed and probed since neither bond pads 13,package substrate 15, nor bond wires 14 obscure access to this region ofIC device 11.

There are several problems with the design of IC device 11 and itsmethod of packaging. One problem stems from the fact that as the densityand complexity of IC device 11 increases, so must the number of bondpads required to control the functions of IC device 11. However, thereis only a finite number of bond pads 13 which can fit along theperiphery of IC device 11. One way to fit more bond pads along theperiphery of the IC device is to increase the overall size of the devicethereby increasing its peripheral area. Unfortunately, this alsosignificantly increases the IC manufacturing costs. Another problem withIC device 11 is that the active circuitry within region 12 must berouted to the peripheral region of IC device 11 in order to beelectrically coupled to bond pads 13. By routing these interconnectlines over this relatively long distance across the IC, the increasedresistive, capacitive, and inductive effects of these lengthyinterconnect lines result in speed reduction of the IC device. Inaddition, the inductance of the bond wires 14 will also severely limitthe high frequency operation of IC devices in these packages.

Techniques have been employed to overcome these and other limitations ofthe design and packaging of IC device 11. FIG. 2a illustrates a top sideview of IC device 20. As illustrated in FIG. 2a, bond pads 21 have beenformed along the top of the entire IC device so that the bond pads nowreside directly over the active circuitry region of IC device 20. Byforming bond pads in both the center and periphery of IC device 20, morebond pads can be placed across the surface of the device than can beplaced only within the peripheral region. In addition, active circuitrywhich underlies bond pads 21 of IC device 20 can be directly coupled toits nearest bond pad using relatively short interconnect lines. Thisminimizes the resistive, capacitive, and inductive effects associatedwith routing interconnect lines over long distances, improving speedperformance.

FIG. 2b is an illustration of a cross-section of IC device 20 afterbeing mounted to a package substrate 22. In order to mount IC device 20to package substrate 22, solder balls 24 are placed on each of bond pads21 to electrically couple each bond pad 21 to its corresponding pad onpackage substrate 22. Each corresponding pad on package substrate 22 is,in turn, coupled to an external pin 23. Note that IC device 20 ismounted to package substrate 22 with its top side facing towards thepackage substrate. In contrast, IC device 11 of FIG. 1b is mounted topackage substrate 15 with its top side facing away from the packagesubstrate. In other words, in comparison to the method used to mount ICdevice 11 to its package substrate 15, IC device 20 is "flipped." Forthis reason, the design of IC device 20 illustrated in FIG. 2a and it'ssubsequent packaging method illustrated in FIG. 2b is referred to asflip-chip technology. The technology is also known as controlledcollapse chip connection (C4), named after the package mountingtechnique of using solder to replace bond wires.

As can be seen in FIGS. 2a and 2b, the top of IC device 20 is obscuredby bond pads 21, solder balls 24, and package substrate 22. Such is thecase for all IC devices packaged using C4 technology. Therefore, it isimpossible to probe the circuitry of IC device 20 in the conventionalmanner described above since the circuitry of IC device 20 cannot beaccessed from its top side.

Alternative techniques have been employed to permit access to theinterconnect lines on top of IC device 20 so that these lines can beprobed. One technique involves redesigning the C4 IC device so that itcan be packaged in a conventional wire bond package. The redesigned wirebond packaged IC device may then be probed from the top of the siliconsubstrate in a more conventional manner. Unfortunately, the C4 IC deviceredesigned for wire bond packaging functions differently in a wire bondpackage than it would in its intended C4 package. As a result, thedebugging process is hindered by the fact that electrical data collectedduring probing of the redesigned C4 IC device may not accurately reflectactual performance of the device when packaged in its native, C4 packageenvironment.

A method is needed whereby a C4 IC device can be probed while in itsnative C4 package environment. This would allow electrical data to becollected from the IC device which reflects the true performance of thedevice as it was intended to operate.

SUMMARY OF THE PRESENT INVENTION

An apparatus is disclosed. In one embodiment, the apparatus includes asemiconductor substrate and a second substrate. The semiconductorsubstrate has a top side and a bottom side. The semiconductor substratehas an integrated circuit and at least one alignment fiducial formed onthe top side. The alignment fiducial is aligned with the integratedcircuit and the alignment fiducial is accessible from the bottom side.The semiconductor substrate further includes a first set of bond pads onthe integrated circuit, the bond pads on the top side. The secondsubstrate has a second set of bond pads corresponding to the first setof bond pads. The semiconductor substrate is coupled to the secondsubstrate at a plurality of solder interconnections disposed between thefirst and the second set of bond pads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is an illustration of an IC device intended for wire bondpackaging.

FIG. 1b is an illustration of a cross-sectional view of the IC device ofFIG. 1a after the device has been packaged.

FIG. 2a is an illustration of an IC device intended for C4 packaging.

FIG. 2b is an illustration of a cross-sectional view of the IC device ofFIG. 2a after the device has been packaged.

FIG. 3a is an illustration of a cross-sectional view of an IC device ina C4 package.

FIG. 3b is an illustration of a cross-sectional view of the C4 packagedIC device of FIG. 3a after a bottom portion of the silicon substrate ofthe IC device has been globally thinned.

FIG. 3c is an illustration of a surface view of the C4 packaged deviceof FIG. 3a after a bottom portion of the silicon substrate of the ICdevice has been globally thinned.

FIG. 3d is an illustration of a surface view of the C4 packaged ICdevice of FIG. 3c after alignment holes have been etched to expose threefiducials.

FIG. 3e is an enlarged view of a fiducial shown in FIG. 3d.

FIG. 3f is an illustration of a cross-sectional view of the C4 packagedIC device of FIG. 3c after a bottom portion of the silicon substrate ofthe IC device has been locally thinned.

FIG. 3g is an illustration of a cross-sectional view of the IC device ofFIG. 3f within the locally thinned region.

FIG. 3h is an illustration of a cross-sectional view of the IC device ofFIG. 3g after probe holes have been etched.

DETAILED DESCRIPTION

A method of probing a C4 packaged integrated circuit (IC) device isdescribed. In the following description, numerous specific details suchas etch depths, process sequences, material compositions, etc. are setforth in order to provide a more thorough understanding of the presentinvention. However, it will obvious to one skilled in the art that thepresent invention may be practiced without employing these specificdetails. In other instances, well-known processes and processingtechniques and equipment have not been described in detail in order toavoid unnecessarily obscuring the present invention.

While diagrams representing an embodiment of the present invention areillustrated in FIGS. 3a-h, these illustrations are not intended to limitthe invention. The specific processes described herein are only meant tohelp clarify an understanding of the present invention and to illustratean embodiment of how the present invention may be implemented in orderto achieve a desired result. For the purposes of this discussion, asemiconductor substrate is a substrate comprising any material ormaterials used in the manufacture of a semiconductor device. A substrateis a structure on which or to which a processing step acts upon.

There are two relatively distinct aspects to the present invention whichwork together in order to allow a practitioner to probe an IC device (or"chip") from the bottom of the semiconductor substrate upon which the ICis formed. First, a method is proposed which allows virtual navigationthrough the circuitry of the chip from the bottom of the chip. Thenavigational method offered herein permits a practitioner to accuratelydetermine a point on the bottom of the chip residing directly below acorresponding point in the circuitry on top of the chip which thepractitioner desires to probe (the probe point). Second, varioustechniques are offered herein whereby a hole is etched through thebottom of the chip in order to allow probing of the probe point.

FIG. 3a illustrates a cross-section of a C4 packaged chip, similar tothe C4 packaged chip described above in conjunction with FIGS. 2a and2b. Chip 40 has an integrated circuit formed on its top side, abovewhich, bond pads have been created. Each of these bond pads on top ofchip 40 are electrically coupled to a corresponding pad on packagesubstrate 43 through a solder ball 42. Each corresponding pad on packagesubstrate 43 is, in turn, coupled to an external pin 44. These pins maybe inserted into an appropriate socket for operation of the IC device.

The thickness of chip 40 is in the range of approximately 400-700microns, but it is only the upper 10 to 15 microns of chip 40 in whichactive circuitry resides. It is this active circuitry which is to beprobed in accordance with the present invention. The remainder of chip40 primarily comprises an inactive silicon substrate. To shorten theamount of time it takes to etch holes through the bottom of thisunderlying, thick silicon substrate (as described in greater detailbelow), the first step in accordance with the present invention is toglobally thin chip 40 by mechanical polishing.

In this first step, the majority of the silicon substrate of chip 40 isremoved from the bottom of the chip by the mechanical polishing process.FIG. 3b illustrates the cross-section 30 of FIG. 3a after chip 40 hasbeen globally thinned to a thickness within the range of approximately50-150 microns. In accordance with the present invention, the siliconsubstrate of chip 40 is etched back to be as thin as possible withoutbreaking or otherwise significantly impacting the performance of thecircuitry on top of the chip. Alternatively, the silicon substrate ofchip 40 may be etched back by chemical mechanical polishing, reactiveion etching (RIE), plasma etching, wet etching, or any combination ofthese techniques. Where the semiconductor substrate of chip 40 comprisesa semiconductor material other than silicon, the appropriate etchchemistries are employed.

In alternate embodiments, globally thinning chip 40 may adversely impactoperation of the IC device, affecting, for example, speed, latch-upcharacteristics, or thermal properties. In addition, other IC's ordiscrete components may be mounted to the bottom of chip 40, makingglobal thinning impossible. Furthermore, other special materials may beformed on the bottom of the chip to aid in the manufacturing oroperation of the IC device. In such cases, the step of globally thinningchip 40 by etching the bottom of the silicon substrate is skipped, andthe practitioner may proceed directly to the next process stepillustrated in FIG. 3d.

FIG. 3c illustrates a surface view 31 of the bottom of the C4 packagedchip 40 having the cross section 30. To ensure proper electricalcoupling between package pins 44 and the circuitry of chip 40, the bondpads on top of chip 40 are aligned to the corresponding pads on packagesubstrate 43 so that the solder balls 42 can properly couple the twotogether. Package fiducials 33 are alignement marks which have beenformed on the package substrate and are aligned to the pads on packagesubstrate 43. Therefore, by aligning chip 40 to the package substratepads, chip 40 is aligned to package fiducials 33 and to the x-ycoordinate system established by these package fiducials.

The x-y coordinate system illustrated in FIG. 3c is used in conjunctionwith a circuit diagram of chip 40 to roughly locate points along thebottom surface of chip 40 which lie directly below corresponding,determinable points in the circuitry on top of chip 40. For example,when chip 40 is accurately aligned to package fiducials 33, chip 40 isoffset from both the x-axis and the y-axis by a known amount. The x andy coordinates of a point on the bottom of chip 40 are determined byprecise measurement from the x and y axes. The known axes offset valuesare then subtracted from the measured x-y coordinates of the point.Then, by referencing the resulting coordinates on a circuit diagram ofchip 40, one can determine the corresponding point in the circuitry ontop of chip 40 which resides directly above the point on the bottom ofchip 40. Of course, this positioning scheme may be worked in reverse tolocate a point on the bottom of chip 40 which resides directly below acorresponding point in the circuitry on top of chip 40. Thus, one cannavigate through the circuitry on top of chip 40 by coordinatedpositioning along the bottom of chip 40.

Unfortunately, due to current package technology limitations, chip 40 isnot always accurately aligned to package fiducials 33, so the offset ofchip 40 from the x and y axes is not a constant. Rather, this offset isknown to fall within a range of values which may vary by as much as 200microns or more. Therefore, the method described above of locating apoint on the bottom of chip 40 residing directly below a correspondingpoint in the circuitry on top of chip 40 may only be used as anapproximation. Alternatively, points on the bottom of chip 40corresponding to points in the circuitry on top of chip 40 may bedetermined by measurement directly from the edge of chip 40. However,since the edge of chip 40 is non-uniform, this method may only be usedas an approximation as well.

The next step in accordance with the present invention is to use the x-ycoordinate system established by package fiducials 33 in the mannerdescribed above to determine the approximate location of features onchip 40 known as chip fiducials. Chip fiducials are alignment marksdesigned into the first metal layer (M1) of the chip and, in accordancewith the present invention, are placed in at least three corners of thechip. Chip fiducials are used in a manner similar to package fiducials33, but since chip fiducials are located within the circuitry on top ofchip 40, they are self-aligned to the circuitry on chip 40 therebyallowing more precise navigation from the bottom of the chip.

Alternatively, chip fiducials may be formed from other layers of thechip. For example, chip fiducials may be formed from field oxide,diffusion regions, polysilicon, or any other interconnect layer. It maybe beneficial for the chip fiducials to be formed from the samesemiconductor layer which the practitioner desires to probe. One reasonfor this is to avoid errors in locating the probe point resulting frommisalignment between semiconductor layers of the IC. Another reason isthat the vertical distance to the probe point may be determined usingcoordinate information from chip fiducials formed in the same plane asthe probe point (described in more detail below). Also, in an alternateembodiment, only two chip fiducials are placed on the chip. These twofiducials are placed in opposite corners of the chip to accommodatealignment of the chip to its circuit diagram.

Using the x-y coordinate system established by package fiducials 33, theapproximate locations on the bottom of chip 40 residing directly belowthe three M1 chip fiducials on top of chip 40 are determined. Oncedetermined, a gas-assisted laser etching system is used to etchalignment holes through the bottom of chip 40 at these locations toexpose the three M1 chip fiducials on top of the chip. FIG. 3dillustrates the C4 packaged chip of FIG. 3c after the underlying siliconsubstrate has been etched, and the three M1 chip fiducials 35 have beenexposed in accordance with the present invention. FIG. 3e is an enlargedview of a fiducial 34 shown in FIG. 3d.

The gas-assisted laser etching system uses an etch chemistry having ahigh selectivity of silicon over silicon dioxide. In this manner, afteretching the silicon substrate from alignment hole regions 34 through thebottom of chip 40, the system stops at the silicon--silicon dioxideinterface at the top of chip 40. Note that in some cases it may benecessary to form a large metal barrier in a metal layer above the M1chip fiducial to shield the laser beam from penetrating all the way topackage substrate 43 through the silicon dioxide in alignment hole 34.Alternatively, other etching techniques may be used to etch alignmenthole 34. For example, one or more alignment holes may be etched bypatterning and chemically etching the bottom of chip 40 using a wet ordry etch. Laser ablation or focused ion beam (FIB) etching may also beemployed, however, these methods may not accommodate end point detectionmethods.

As stated above, due to the misalignment between chip 40 and packagefiducials 33, only the approximate location of chip fiducials 35 can bedetermined from the bottom of chip 40 using package fiducials 33 asreference. Therefore, it is necessary that chip fiducials 35 andalignment holes 34 be large enough to account for any possiblemisalignment between chip 40 and package fiducials 33. However, thereare significant trade-offs to consider.

For example, depending on the etching method used, it can take longer toetch a larger alignment hole than a smaller alignment hole. As a result,increasing the size of the alignment hole increases the throughput timeof probing the chip. Also, by etching large holes into the siliconsubstrate, the performance of the chip may be affected, so electricaldata gathered by the probing method of the present invention may notaccurately reflect natural operation of the chip in its nativeenvironment. In addition, the chip may be destroyed if the alignmenthole is made large enough to etch through a nearby diffusion region orother active circuitry of the chip. Therefore, it is currently desirableto keep alignment hole 34 as small as possible while still being able tolocate chip fiducial 35 from the bottom of chip 40 using packagefiducials 33 for guidance.

While increasing the size of chip fiducial 35 improves a practitioner'sability to locate the fiducial, increasing the size also reduces theavailable area on top of chip 40 where circuitry can be formed.Therefore, it is also desirable to keep chip fiducial 35 as small aspossible while still being able to locate this fiducial from the bottomof chip 40 using package fiducials 33 for guidance. In one embodiment,chip fiducials 35 are each approximately 150 microns end to end, andalignment hole 34 is approximately 50 by 50 microns. As alignmentaccuracy between chip 40 and package fiducials 33 improves, chipfiducial 35 and alignment hole 34 may be proportionately minimized insize since the accuracy in locating the chip fiducial from the packagefiducials will proportionately improve. Alternatively, alignmentaccuracy may advance to the level where the probe point may be locateddirectly from the package fiducials.

Once the silicon substrate has been removed from alignment hole 34 ofchip 40, M1 chip fiducial 35 can be viewed through the transparentsilicon dioxide film separating M1 from the silicon substrate. Chipfiducials 35 are then used in conjunction with a circuit diagram of chip40 to more precisely locate the point on the bottom of chip 40 whichresides directly below the probe point in the circuitry on top of chip40 (described in more detail below). In an alternate embodiment, only asmall, discernible portion of a chip fiducial need be exposed through analignment hole. Only enough of the chip fiducial needs to be visiblethrough the alignment hole to allow a practitioner to identify andlocate the corresponding portion of the chip fiducial on a circuitdiagram of chip for use in referencing the circuit diagram to the actualchip.

Once three alignment holes 34 have been etched to expose M1 chipfiducials 35 through the bottom of chip 40, as illustrated in FIG. 3d,M1 chip fiducials 35 are used to enable virtual navigation through thecircuitry of chip 40 from the bottom of the chip. The C4 packaged chipof FIG. 3d is affixed to a high precision, computer controlledpositioning stage. Then, using chip fiducials 35 for alignment, acomputer-based circuit diagram of chip 40 is electronically merged withthe stage position of chip 40 on the computer's display screen. As aresult, one can accurately determine the location of any point on thebottom of chip 40 residing directly below its corresponding point in thecircuitry on top of chip 40 by referencing the computer-based circuitdiagram displayed on the computer screen. Thus, a virtual navigationmethod is enabled. Note that this navigational technology is currentlyavailable on analytical equipment such as FIB etchers, but hasheretofore been limited to top side navigation only.

While only two chip fiducials 3 may be required to align the chip'scircuit diagram to the backside of the chip, a third fiducial is used tocorrect for tilt angle and backside thickness non-uniformities of chip40. The plane defined by these three M1 chip fiducials establishes theplane in space in which the M1 layer of chip 40 resides. It is withinthis plane that the desired probe point is located. The M1 plane definedby the three M1 chip fiducials is used to determine how deep a hole mustbe etched through the bottom of chip 40 before reaching the desiredprobe point in the first metal layer of the chip. In other words, whilealigning a circuit diagram to two chip fiducials provides for twodimensional x and y navigation, a third fiducial introduces a vertical zcoordinate for three dimensional navigation. Depth of focus measurementsare used to measure distances in the z direction.

Alternatively, two or more chip fiducials may be used to define a twodimensional coordinate system for navigation. In this embodiment, properetch depths may be determined by, for example, an etch endpointdetection technique, timed etch, or depth measurement comparisons toknown depths. In an alternate embodiment, any number of fiducials orother alignment marks are revealed by etching through the bottom of chip40 in order to establish a coordinate system. In embodiments of thepresent invention in which a probe point resides in a layer other thanthe M1 layer, fiducials in this alternate layer are revealed in asimilar manner. These fiducials are then similarly aligned to circuitdiagrams of the chip in order to establish a navigational systemsubstantially as described above. Also, fiducials in a first layer maybe aligned to a circuit diagram in a method for locating and probing aprobe point residing in the first or a second layer.

Note that the circuit diagrams used for navigation from the bottom ofchip 40 must be "flipped" with respect to conventional circuit diagrams.In accordance with the present invention, navigation is performed byaligning the underside (or flip-side) of the circuitry formed on chip 40to the flip-side of its respective circuit diagram. In contrast,traditional circuit location methods align the circuitry on top of thechip to its conventional, top-side circuit diagram.

As an alternative to the computer-aided, virtual navigation techniquedescribed above, M1 chip fiducials 35 may be used to establish an x-ycoordinate system just as package fiducials 33 were used. The x-ycoordinate system established by chip fiducials 35 is used inconjunction with a circuit diagram of chip 40 to accurately locatepoints along the bottom surface of chip 40 which lie directly belowcorresponding, determinable points in the circuitry on top of chip 40.For example, the x and y coordinates of a point on the bottom of chip 40can be determined by precise measurement from the x and y axes. Bycomparing these coordinates to a circuit diagram of chip 40, one candetermine the corresponding point in the circuitry on top of chip 40which resides directly above the point on the bottom of the chip 40. Asbefore, this scheme may be worked in reverse to locate a point on thebottom of chip 40 which resides directly below a corresponding point inthe circuitry on top of chip 40. Thus, one can navigate through thecircuitry on top of chip 40 by coordinated positioning along the bottomof chip 40.

Using the virtual navigation method described above, the point on thebottom of chip 40 that resides directly below the probe point in the M1layer on top of chip 40 is located. Once located, this region is locallythinned using a gas-assisted laser etching system. Local thinning is aprocess by which the majority of the remaining silicon substrate isremoved in the local region underneath the probe point. FIG. 3fillustrates a cross-sectional view of the C4 packaged chip of FIG. 3dafter region 45 has been locally thinned using a gas-assisted laseretching system.

The size of locally thinned region 45 depends on the size and locationof the probe point within the circuitry on top of chip 40. In accordancewith the present invention, to probe an M1 interconnect lineapproximately 1 micron wide, locally thinned region 45 is in the rangeof approximately 100 by 100 microns to 300 by 300 microns centered aboutthe location directly beneath the probe point. However, because multipleprobe points may be accessed by etching multiple probe holes through thelocally thinned region, one may wish to thin a larger region to gainaccess to multiple probe points which may be a large distance apart fromeach other. For example, a region greater than 500 by 500 microns may belocally thinned where one desires to probe individual interconnect lineswhich are approximately 500 microns apart.

The sidewall profile of locally thinned region 45 depends on the methodused to etch this region and the practitioner's requirements. Inaccordance with the present invention as illustrated in FIG. 3f, thinnedregion 45 has graded sidewalls formed by etching out layers ofprogressively smaller sections of the silicon substrate using thegas-assisted laser etching system. The dimensions of the largest,outermost section in the first layer etched from region 45 isapproximately 200 by 200 microns. The dimensions of the smallest,innermost section in the final layer etched from region 45, closest tothe top of chip 40, is approximately 100 by 100 microns.

FIG. 3g illustrates an enlarged view of locally thinned region 45 of theC4 packaged chip of FIG. 3f. The graded, stair-step profile, 54, formedby the gas-assisted laser etching method described above is illustratedwhere each step represents a layer of progressively smaller sections ofthe silicon substrate which have been etched from the region. In analternate embodiment, where local thinning is accomplished by otherlaser techniques, patterning and chemically etching the region, FIBetching, or any combination of techniques, the sidewalls may be graded,vertical, or undercut. In general, the size, shape, and sidewall profileof the locally thinned region should be selected to minimally interferewith device performance, accommodate subsequent probe hole formation,and permit probing of the circuitry.

Cross-sections of three M1 interconnect lines 49 are illustrated in theenlarged view of locally thinned region 45 in FIG. 3g. These threeinterconnect lines represent the probe points to be probed in accordancewith the present invention. Layer 47 comprises an electricallyinsulating silicon dioxide material used to isolate M1 interconnectlines 49 from each other, from the underlying silicon substrate 46, andfrom the rest of the circuitry of chip 40. The remaining layers of theintegrated circuit, including additional metal interconnect anddielectric layers, reside in region 48.

Depth 50 is the distance from the M1 layer to the opening of locallythinned region 45. Depth 50 is in the range of approximately 3 to 25microns. Therefore, it is necessary to etch region 45 to withinapproximately 3 to 25 microns below the desired probe point in the M1layer of chip 40. The three dimensional navigation method describedabove makes this possible. By using the three M1 chip fiducials todefine the plane in which the M1 layer of the chip resides in an xyzcoordinate system, region 45 can be etched to a depth (z coordinate)within the required range. Locally thinned region 45 is not etched allthe way through silicon substrate 46 to silicon dioxide layer 47 becausethis would destroy electrically active regions within silicon substrate46 such as diffusion regions. Alternatively, in an embodiment in whichthe location of the M1 plane is not defined, the proper depth of thinnedregion 45 may be achieved by using an etch endpoint detection technique,timed etch, depth measurement comparisons to known depths, or anycombination thereof.

In order to probe M1 interconnect lines 49, probe holes are etchedthrough silicon substrate 46 and through a portion of silicon dioxidelayer 47 to allow electron-beam ("e-beam") probing of interconnect lines49 from the bottom of chip 40. Therefore, in order to prevent theintegrated circuit from being destroyed during etching of these probeholes, there is no active circuitry underneath MI interconnect lines 49at the probe points. In other words, the portions of interconnect lines49 to be probed reside over isolation regions or other inactive regionsof the integrated circuit. Probe holes etched through isolation or otherinactive regions will not significantly disrupt performance of the chip.

The location of a probe hole must be determined with much higherprecision than the location of locally thinned region 45 since animproperly placed probe hole can destroy the chip if etched through animportant IC component such as a transistor. Therefore, an FIB etchingsystem is used to etch these probe holes. Typical FIB etching systemsuse highly precise electron microscopy methods to generate images andcoordinate information from the substrate being etched. In accordancewith the present invention, after thinning region 45 in a gas-assistedlaser etching system, the chip is transferred to an FIB system and thecircuit diagram of chip 40 is realigned to the M1 fiducials using themore precise electron microscopy methods available in FIB systems.

Unfortunately, electron microscopes cannot resolve images through asilicon dioxide layer. Therefore, in order to align the circuit diagramof chip 40 to its M1 fiducials in an FIB system, the alignment holesneed to be further etched through the silicon dioxide insulation layerto expose portions of the M1 fiducials. Etching of the alignment holesis accomplished using the FIB etching system. Once exposed, theseportions of the M1 fiducials are used substantially as described aboveto define the plane in which the M1 layer of the chip resides in an xyzcoordinate system. By electronically overlaying and aligning thecomputer-based circuit diagram of chip 40 to the M1 fiducials, virtualnavigation through the circuitry of the chip is again enabled. Usingthis method, the point within locally thinned region 45 residingdirectly beneath the desired probe point of the circuit can be moreaccurately determined.

FIG. 3h illustrates the substrate of FIG. 3g after two probe holes 52and 53 have been etched using a focused ion beam etching system tofinally access the probe points of interconnect lines 49. Where onedesires to probe only a single metal line, as illustrated by probe hole52, the probe hole is etched having a diameter 51 in the range ofapproximately 2 to 6 microns. Where one desires to access multiple metallines within a single probe hole, as illustrated by probe hole 53, awider probe hole may be etched, taking care not to etch through anyelectrical components of the circuit.

Alternatively, the diameter 51 of a minimally sized probe hole 52 may bereduced below 2 microns, however, a practitioner must consider thefeasibility of probing metal interconnect lines through high aspectratio probe holes. Generally, a probe hole aspect ratio (depth/width) ofless than three sufficiently allows a metal interconnect line to beprobed by an e-beam probing system. Current e-beam probing technologydictates that the width of an interconnect line 49 at the probe point beat least 1 micron to obtain reliable electrical information from theinterconnect line. Naturally, narrower lines may be probed as probingand etching technologies advance.

In an embodiment in which the location of the M1 plane is not defined,probe holes 52 and 53 may be etched using an etch endpoint detectiontechnique, timed etch, depth measurement comparisons to known depths, orany combination thereof. For example, one may detect when the etch ofprobe holes 52 and 53 reach the interface between silicon substrate 46and silicon dioxide layer 47, and provide sufficient over-etch to accessM1 interconnect lines 49. Note that the proffered method of etchingprobe holes to M1 interconnect lines may be employed for etching probeholes to alternate regions of the circuit as well.

For example, in one embodiment, a diode diffusion region coupled to anM1 interconnect line is probed. Many such diffusion regions may alreadyexist in typical IC devices to, for example, protect gate oxide damageduring the manufacturing process. In this embodiment, the IC is alignedto fiducials in the diffusion region and the probe hole is etched to thediffusion region probe point substantially as described above. Inanother embodiment, the probe point resides in a polysilicon or othermetal layer of the chip. Higher metal layers of the chip are accessed insituations where the M1 line is too small or otherwise inaccessible fromthe bottom of the chip. In such embodiments, the IC may be speciallydesigned to accommodate bottom-side probing. Also, in an alternateembodiment, a laser etching system or other etch technique such as, forexample, photomask and dry etch, may be used to etch probe holes.

Once the underside of metal interconnect lines 49 have been exposed byprobe holes 52 and 53, an e-beam probing system is used to obtainelectrical data from M1 interconnect lines 49 through the bottom of thechip while operating the chip through the C4 package from the top.Alternatively, mechanical, laser, or infrared probing may be used toobtain electrical data from the probe point.

Thus a method for electrically probing an integrated circuit from thebottom of the chip has been described. This method may be found usefulin debugging applications where access to the IC device is obscured by,for example, the package technology employed.

What is claimed is:
 1. An apparatus comprising:a semiconductor substratehaving a top side and a bottom side, said semiconductor substrate havingan integrated circuit and at least one alignment fiducial formed on saidtop side, said alignment fiducial being aligned with said integratedcircuit, said alignment fiducial being accessible from said bottom side,said semiconductor substrate further comprising a first set of bond padson said integrated circuit, said bond pads on said top side; and asecond substrate having a second set of bond pads corresponding to saidfirst set of bond pads, said semiconductor substrate coupled to saidsecond substrate at a plurality of solder interconnections disposedbetween said first and second set of bond pads.
 2. The apparatus ofclaim 1 wherein said semiconductor substrate comprises at least twoalignment fiducials.
 3. The apparatus of claim 1 wherein saidsemiconductor substrate comprises at least three alignment fiducials. 4.The apparatus of claim 1 wherein said alignment fiducial is locatedwithin a diffusion region of said integrated circuit.
 5. The apparatusof claim 1 wherein said alignment fiducial is located within a metallayer of said integrated circuit.
 6. The apparatus of claim 1 whereinsaid semiconductor substrate comprises a silicon substrate and saidintegrated circuit comprises at least one interconnection metal layerhaving been formed over said silicon substrate.
 7. The apparatus ofclaim 6 further comprising a barrier metal layer having been formedabove said interconnection metal layer.
 8. The apparatus of claim 1wherein said second substrate comprises a C4 package.
 9. The apparatusof claim 1 wherein said second substrate comprises a printed circuitboard.
 10. An apparatus comprising:a semiconductor substrate having atop side and a bottom side, said semiconductor substrate having anintegrated circuit and at least one alignment fiducial formed on saidtop side, said alignment fiducial being aligned with said integratedcircuit, said alignment fiducial being accessible from said bottom side,said semiconductor substrate further comprising a first set of bond padson said integrated circuit, said bond pads on said top side; and asecond substrate having a second set of bond pads corresponding to saidfirst set of bond pads, said semiconductor substrate coupled to saidsecond substrate at a plurality of solder interconnections disposedbetween said first and second set of bond pads, said second substratefurther comprising at least one alignment mark that is aligned with saidsecond set of bond pads.
 11. The apparatus of claim 10 wherein saidsemiconductor substrate comprises two alignment fiducials.
 12. Theapparatus of claim 10 wherein said semiconductor substrate comprisesthree alignment fiducials.
 13. The apparatus of claim 10 wherein saidsecond substrate comprises two alignment marks.
 14. The apparatus ofclaim 10 wherein said second substrate comprises three alignment marks.15. The apparatus of claim 10 wherein said alignment fiducial is locatedwithin a diffusion region of said integrated circuit.
 16. The apparatusof claim 10 wherein said alignment fiducial is located within a metallayer of said integrated circuit.
 17. The apparatus of claim 10 whereinsaid semiconductor substrate comprises a silicon substrate and saidintegrated circuit comprises at least one interconnection metal layerhaving been formed over said silicon substrate.
 18. The apparatus ofclaim 17 further comprising a barrier metal layer having been formedabove said interconnection metal layer.
 19. The apparatus of claim 10wherein said second substrate comprises a C4 package.